PROJECT II (Kl: SEMINARA) Over the last five years, Project II has explored the physiology of kisspeptin as a powerful stimulus of GnRH secretion. The focus of Project II in the current grant cycle is the integration of kisspeptin with another neuropeptide that has recently been implicated in the genetics of GnRH deficiency, neurokinin B. Patients with mutations in the neurokinin pathway have high rates of reversal of their hypogonadotropism with restoration of endogenous GnRH-induced LH pulses. The first specific aim explores the subphenotype of reversible GnRH deficiency in patients with mutations in the kisspeptin and neurokinin B pathways, and in particular, the relationship between reversal and sex steroid exposure. The second specific aim explores the physiologic interplay between kisspeptin and neurokinin B in stimulating GnRH secretion in normal volunteers and patients carrying mutations in each of these signaling pathways. These tools will include continuous kisspeptin administration via infusion-(acting as a surrogate kisspeptin receptor antagonist), and a neurokinin B receptor antagonist. The third specific aim extends the physiologic tools used in the human into the mouse, as well as incorporating phenotyping of mice with targeted deletions within the neurokinin B pathway, and developing a mouse model of reversible hypogonadotropism.